State of the science: heart rate variability in health and disease

HRV is a non-invasive tool to assess cardiovascular autonomic function. Fundamental to HRV is a determination of the RR interval. This is the time (typically in milliseconds) between two successive heart beats, measured from one R peak to the next. Variations in RR intervals yield measures of HRV. Heart rate (HR) increases and decreases as one engages and disengages from physical activity. Even for an individual at rest, HR will vary from one beat to the next. This variation is termed HRV. In large population studies, greater variability is consistently associated with better health. HRV has been used in clinical and sports medicine, for example, in prognostication after myocardial infarction1 and prevention of overtraining in athletes.2 HRV results from multiple complex processes and so caution is required in interpretation. Several of these processes involve the central nervous system and include autonomic outflow, peripheral nerve conduction; autonomic receptors; intracellular signal transduction and end-organ responsiveness.3 The almost constant HR after cardiac transplantation (HRV is practically lost) suggests that the autonomic nervous system is the single most important factor mediating HRV, with respiratory sinus arrhythmia, blood pressure variability and circadian rhythms as major contributors.3 HR responds more slowly to sympathetic than parasympathetic inputs with sympathovagal balance also important. The typical way to determine HRV is through a single lead ECG. For short-term measurement, 5 min recordings are standard. The recording is then input to software which can locate R peaks. These are marked on the computer and examined visually to correct any artefacts or ectopic beats errors. Corrected RR intervals are sometimes classified as Normal to Normal (NN) intervals. The software can then output these RR intervals as a series of …